Playing with the XrmToolbox version (Beta 188.8.131.52).
Small UX tweak, would be nice if the ribbon workbench had a "close tab" button in its own ribbon (within the XrmToolBox).
Thanks for the suggestion!
You can use Tabs→Close Current Tab
Since the RWB has the same UI in both the installable solution and the XrmToolbox I’d rather not do add a button in one and not the other.
Greg Owens commented
This is particularly a problem since it is not possible to change the connection on an existing RibbonWorkbench tab.